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Moment about an axis

  1. Nov 30, 2015 #1
    1. The problem statement, all variables and given/known data
    the homogenous plates shown in the figure has a mass of 10kg and is subjected to a forec and moments alonf its edges. If it is supported in the horizontal plate by means of a roller at A , a ball and a socket joint at B , and cord at C , determine the components of reaction at the supports .

    https://www.physicsforums.com/attachments/3233-png.92686/

    2. Relevant equations


    3. The attempt at a solution
    I wanna ask about the total moment about y ' axis ,
    it's -300(1.5) + 981(1.5) -200 +Tc(3) = 0
    the 2ooNm is acted along the y axis , right ? why we should consider it at y 'axis ?

    The moment about y given is 300(1.5) +981(1.5) –Bz(3) –Az(3) -200Nm = 0
    i can undersatnd for y axis , but not for y ' axis ...
     
    Last edited by a moderator: Dec 1, 2015
  2. jcsd
  3. Nov 30, 2015 #2

    SteamKing

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    Thanks for posting the larger image. It makes things much easier to read.

    As far as the 200 N-m moment shown on the y-axis in the figure, this is what is known as a couple, or a free moment. A couple can be moved at will, and as long as it is included in the sum of the moments for a particular body, the equilibrium of the body is not disturbed. In other words, it doesn't matter where the couple is located; it will always have the same magnitude and direction.
     
  4. Nov 30, 2015 #3
    how do u know that it's a couple ? it's not stated there . If it is , why cant we include the 200Nm in the calculation of moment at x and x' axis ?
     
  5. Nov 30, 2015 #4

    SteamKing

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    Well with units of N-m, it's not a force.

    The dirty secret about moments and couples is they're also vector quantities, like forces. A couple has a magnitude and a direction.

    For this couple, it's shown acting along the y-axis, not the x-axis or the z-axis. When it comes time to sum moments, the magnitude of this couple is supposed to be included with any other moments acting about the y-axis.
     
  6. Nov 30, 2015 #5
    but y and y ' axis are different ? why we have to include the 200Nm which is on y axis to y' axis ?
     
  7. Nov 30, 2015 #6

    SteamKing

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    Because you can't simply ignore the couple just because you are using a different axis about which to calculate moments.
     
  8. Nov 30, 2015 #7
    sorry , i still dont gt you , can you explain further ? the question doesnt state the 200Nm is a couple moment , so the 200Nm is just a moment , right ?
     
  9. Nov 30, 2015 #8

    SteamKing

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    Well, it's not the moment of any force which I can see on the diagram, so by the process of elimination, it must be a couple. Whatever you want to call it, a couple, Cyril, or a demiflitchit, you still can't ignore it in your calculations.
     
  10. Nov 30, 2015 #9
    a moment must be a couple moment ?
     
  11. Nov 30, 2015 #10

    SteamKing

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    It doesn't always have to be. Like I said before, you can have the moment of a force, but you can also have a special moment, which is called a couple, which is produced by equal and opposite forces F which are separated from one another by a distance d = 2s.

    Mom_8.gif

    https://en.wikipedia.org/wiki/Couple_(mechanics)

    The moment produced has a magnitude of M = F * 2s = F * d. There is no net force since the Fs are equal in magnitude but acting in opposite directions.

    Statically, the couple created by the two forces acting on the beam above can be located anywhere along the length of the beam without changing the net of the forces and moments acting on the beam.

    If there were only one force acting on the beam above, you could move the force F from the end of the beam to the center and add a couple of magnitude F * s. As far as the statics of the beam is concerned, both situations are equivalent.
     
  12. Nov 30, 2015 #11
    so , the above one is called special moment ? which is also known as a couple ?
     
  13. Nov 30, 2015 #12

    SteamKing

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    Yes. It's also sometimes referred to as a 'free moment'.
     
  14. Nov 30, 2015 #13
    the question just gave the 200Nm acted in negative y direction , how would you know that it's a free vector moment ?
     
  15. Dec 1, 2015 #14
    so , there are 2 types of moment , namely the moment due to force and free vector moment ( which ia couple moment ) ??
     
  16. Dec 2, 2015 #15
    can i explain in this way ? when the -200Nm torque exist at the y axis , it will turn about the y axis , when it rotate , it will somehow rotate in the y' direction too...??
     
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